1. Field of the Invention
The present invention is directed to a circuit arrangement for white level acquisition for the black/white quantization of scan signals of individual photo sensors having different gray levels in which an adder is assigned to the digitized output signals of the photosensor which has a second input connected to the outputs of the white level memory, a second adder whose input links the currently stored white levels with a correction and reads the results signal back into the white level memory and a comparator following the first adder for supplying a black signal or white signal dependent on a threshold derived from maximum blackening values and averaged blackening values of the output signal of the first adder.
2. Description of the Prior Art
There is a problem in optical character recognition that, on the one hand, the recognition unit requires what is referred to as a pixel image wherein every pixel has a black or white level assigned thereto, but that the optical scanner, on the other hand, supplies only an analog video image composed of different gray levels. Great care is therefore to be exercised in the black/white quantization on the basis of an optimally-precise gray level reproduction. Accurate gray level reproduction, however, encounters the following difficulties:
The optical scanner generates a pixel image that is formed of the scan signals of a plurality of photodiodes. In the ideal case, the photodiodes should have the same transfer characteristics, so that the output signal at all pixel diodes would be strictly proportional to the incident light power. The fact is, however, that the signal for at least a part of the diodes is not zero given darkness and that the proportionality factors or, respectively, the sensitivities differ from pixel diode-to-pixel diode.
Actually it is not the light powers incident on the pixel diodes per se that are relevant for the gray levels; rather, it is the reflection factors of the sub-areas of the paper belonging to the pixel. These, however, are falsified in the pixel diode light power, namely, due to non-uniform illumination of the paper and due to non-uniform brightness transmission of the objective lens (for example, "vignetting"). In the ideal case, the raw data of the pixel gray levels must be corrected such that they are proportional to the paper pixel reflection factors of entirely black through entirely white.
The clamp circuit for chronologically variable, digitized input signals disclosed in the German patent 28 13 352 is referenced in this context, whereby the output signals are referred both to a variable reference value derived from an extreme value of the input signals as well as to the output signals themselves. For example, such a clamp circuit is employed in optical character recognition, whereby the evaluation of a scan signal as a white level or, respectively, black level, is only possible with a variable reference value because of various influences, including that of the changing contrast between character and character background. The variation of this reference value is achieved by a constant decrease or by a reacceptance of the reference value per clock step. A white level is thereby stored for every pixel diode, this white level being modified in the following fashion given the serial arrival of a new pixel gray level. When the gray level is whiter than the stored white level, then the gray level is written into the memory as a new gray level. When, by contrast, the gray level is darker than the stored white level, the latter is modified or, respectively, "aged" in the direction towards back by a slight decrement.
On the basis of immediate registration of whiter levels and only slow decrementation in a direction towards black, the function of a maximum value memory derives that contains the whitest level of the recent past for every pixel diode. This method especially has two disadvantages.
A new white level is also registered even if it occurred only briefly. A small, shiny white point on the paper can thereby shift the white level such that the gray level following this pixel diode are referred to an excessively high white level for some time, i.e. are reproduced excessively dark. Blackenings can be drawn "behind" shiny points as artifacts.
When a pixel diode having retarded scan rate is moved over a dark character element, the white level decrementation has adequate time to greatly diminish the white level. Given a slowly scanned or, respectively, stationary character, the impression then arises as though this character were fading.